Separation by crystallization



June 14, 1960 1 B. cRoLEY SEPARATION BY cRYsTALLIzATIoN 2 Sheets-Sheet 1 Filed Jan. 3, 1955 June 14, 1960 l.. B. cRoLEY SEPARATION BY cRYsTALLIzATIoN 2 Sheets-Sheet 2 Filed Jan. 3, 1955 THM x0 lo r--- \T.

United States Patent O i 2,940,272 SEPARATION BY CRYSTALLIZATION Leo B. Croley, Bartlesville, Okla., assigner to Phillips Petroleum Company, a corporation of Delaware Filed Jau. 3, 1955, Ser. No. 479,310 14 Claims. (Cl. 62-58) This invention relates to separation by crystallization. In one aspect, this invention relates to the separation of components of mixtures by fractional crystallization. ln a more specific aspect, this invention relates to an improved method and means for controlling the separation of components of mixtures by fractional crystallization.

Separations of compounds may be effected by distillation, solvent extraction and crystallization. Although distillation and extraction are generally preferred because of economy and convenience of operation, there are some instances in which such processes cannot be successfully utilized. Many chemical Aisomers have similar boiling points and solubilities `and cannot be separated satisfactorily by distillation or extraction. Separation by means of fractional crystallization can be satisfactorily utilized in many cases in making such separations. Fractional crystallization has one great advantage over other methods of separation in that it is the only separation method which theoretically offers a pure product in a single stage of operation in systems in which the desired component of a mixture solidifies at temperatures above that at which the other components solidify. Thus, whereas distillation and extraction theoretically require infinite stages for a pure product, crystallization in many cases requires only one. This is because of the nature of the phase equilibria in distillation and extraction, while by crystallization, substantially pure crystals can be formed from many solutions in one stage, although the desired component may be of low concentration in the liquid feed.

Crystallization is thus well suited, not only to the separation of many chemical isomers which can be separated by no other means, but also to the purification of many compounds which cannot be economically purified by other means. Whereas one stage of crystallization theoretically offers a pure product, -attainment of this ideal stage rhas been difficult. Complete removal of occluded impurities without substantial loss in yield is required.

In order to separate constituents of mixtures by fractional crystallization, it is necessary to adjust the temperature of the mixture to one which is below the temperature at which crystals of any one of the pure constituents form. For example, when para-xylene is separated from a mixture of isomeric C8 alkyl benzenes, it is necessary to cool the mixture to a temperature in the neighborhood of about \-57 to about -7 8 C.

Methods of separating a pure component from a mixture have been devised whereby the mixture to be separated is introduced into a heat exchange zone wherein a mixture of crystals and liquid is lformed and that mixture is then introduced into an elongated purification chamber through which the crystals are moved as a compact mass. One such means for purifying crystals has been disclosed by l. Schmidt, Re. 23,810.

ln the process disclosed by l. Schmidt, a mass of crystals is moved through an elongated chamber to a melting zone wherein the crystals are melted. A portion of liquid corresponding to the melt is caused to move countercurrently through at least a portion of the crystal mass so as to displace occluded impurities from the crystal mass approaching the melting zone. The exact mechanism whereby this displacing liquid corresponding to the melt improves the purity of the final product is not completely understood. However, it is presently believed' Patented June 14, 1960 roeY .freezes from the reux stream `as it comes -in contact with the cold crystal mass moving toward the melting zone. Thus, the portion of the crystal mass which approaches the melting zone does not contain any appreciable amount of impurities and the resulting product which is removed from the melting zone is of extremely high purity.

lt is highly desirable for the best operation of such a purification system to remove `as much of the mother liquor from the crystals as possible before introducing those crystals into the purification chamber to be cornpacted. ln this manner, a portion of the impurities can be eliminated from the purification system before subjecting the crystals to the final purification step. This is usually accomplished in a preconcentration zone which includes a filtering means for separating crystalline from non-crystalline material.

Several different types of filters can be used in such a preconcentration zone for separating mother liquor from the crystals, such as a rotary lter, a centrifuge or a pressure-type filter in which the crystals are partially compacting means such as an auger or a piston. The operation of the filter and the crystal purification column depends for its efficiency primarily on one variable that is unique in fractional crystallization processes as compared with conventional separation processes, such as fractional distillation or solvent extraction. This variable is the solids content of the partially frozen streams as they pass through the steps of the process. There is no4 known means for conveniently, continuously analyzing suclia partially frozen stream for the percent solids therein and, therefore, line control of a fractional crystallization process is difficult to attain. lt isV desirable to maintainthe solids content at constant values in the various steps of the process in order to aclt'eve the optimum efficiency of which the process is capable. For example, an undue decrease in solids content in a partially frozen stream lowers the effectiveness of the process so that a lower yield of a given purity of product is produced. Eiective operation demands -a system for maintaining the solids content of the various partially frozen streams at constant optimum levels. The most important stream ina fractional crystallization process, in this respect, is the crystal slurry which is passed into the crystal purification column. lt is essential that the solids content of this stream be maintained at a constant level. Since there is no convenient device for continuously indicatingthe solids content of such a stream, a control system for the fractional crystallization process which maintains the solids content constant at the desired level is needed.

' it is an object of this invention, therefore, to provide a system for separating components of mixtures by fractional crystallization. p

It is a further object to provide an improved method for controlling the separation of components of mixtures by fractional crystallization.

It is a still further object to provide a method and means for maintaining the solids content in crystal slurries in a fractional crystallization process at a constant level.

Other objects of the invention will be apparent to those skilled in the art upon study of the Vaccompanying, disclosure and drawings in which:

Figure l is a schematic representation of an embodiment of the fractional crystallization system of this invention; and

Figure 2 is a schematic representation showing modications of the fractional crystallization invention.. 4

In accordance with the invention, the rate of flow of I mQtherliguor-from 4a preconcentration zone in a fractional erystallizatioa process iS maintained in al proso- Y passodlffvom tho Proooiioentration zooo to the orystal nilriiioation ooluoin; thus affording optimum. oloioitoy' v Ain :the procs. Better separation occurs whenthe rate oiilowoi mother liquor ,from the prooonoontratoil ,Zone

. is maintained in a prosoleotod ratio. to the rate of flow of product Vfnznn the crystal purification column, and therefore, this is the preferred embodiment of the inven- Fu'rther in accordancerwith this invention, an anal rcontinuously monitors the composition ofv the Vfeed streamto the process and continuously adjusts the ratio vbetween Vthe yrate of ow of mother liquor Ifrom the preconc'entration zone and the rate of iow of product trom theprocess or the ratio between said mother liquor rate .ofilow and the rate of ow of feed to the process to oomroiisate for ohansos in the food Composition This aids-,in maintaining an even more constant solids content .iu tho crystal slurry passed y frorll tho pfooonoontfation .tono .to the ors/stal Pulriioatioti ooluion- Still further in Yaoomfdfinos with this invention, thorato of flow-of .ro

frigerant 'through the chiller in the preconcentration Yzione is aintained in a preselected ratio to the rate of ow 'of food to tho process to further 'aid in maintaining a .Coil- Ystant Ys olils content in the crystal slurry passed into the crystal purification column. Y

'Although invention iS particularly applioablo to systems which the temperature at which crystals of the pure component form is considerably higher than the temperature to which the component is cooled in liquid mixture to form crystals thereof, this procedure can advantageously utilized in practically any system ritornfihich.. fractional crystallization isV applicable so as to increasethe c flciency ot .the process. This invention is applicable yto tho Sonar-ations in mony multipcomponont s'yslterns Ythe components .of vwhich have practically the boiling point 'and are. .therefore dioult to. Sopo- .rafte by .f .raotioiiol tiistillation. or to mixtures .Wl-'lioh have in the purification of a component of, say 1.5 ,to 25 percent purity, so vas to effect Ia product purity of 98 percent. In order to illustrate some of the .systems to vwhich the invention is applicable, the followl,ingr compounds are =groupedwith respect to their boiling points- .Group A B.P., I`.P.,

Benzene 80 5. 5 nfHexann 69 1r-11er) l e. 98. 5,2V -90 5 Carbon tetrachloride '77 22. 8 Acrylonltrll 79 .-'82 Ethyl alponoL Y 78. 5 .117. s 2,2-Dimeth-yl 79 -125 3,-3gDimethylpentane Y 86 `etliyletlryl `ketone 79. 6 -86. 4 Methyl prop'ionate-- i Y 79. 9 -87. 5 .1Methylatzrylate-. 80. 5

.1 3Cyclnbl3aione 80. 5 -98 itil-Bimotbylpetane --L-- 80. 8 123. 4 80.9 -25 8l.. 4 6. 5 82 -42 sa 103.1 9.0 o 89. 4

system of this Yso Ysistemi.'consisting ff yf/calamiteiten] .otfiw o r'fmore of the components within any .One v-of Ythegroups may "be separatedby the process .of the fmvention, as well 1 Xylonos or from para ,metas and onliol-ztyloiios or from anti ortho-Knows plus ethyloiisis; .av also 'be Y ne.

a mixture. of para?, mota Multi-component systems which may be effectively separated so as to re'cover one or more of the components in substantially pure form include 2,2-dimethy1pentane, 2,4-dimethylpentaue, 2,2,3-trimethylbutan1e, methyl cyclohexane, 2,2,4-trimethylpet1tane, and carbon tetrachloride, chloroform, and acetone. The invention is also applicable to the separation of individual components from a system of cymenes and a system including the xylenes.

This invention can also be utilized to purify naphthalene, hydroquinone, (1,4-benzenediol), paracresol, paradichlorobenzene, and such materials as high melting waxes, fatty acids, and high molecular weight normal parains. The system can also be used to separate anthracene, phenanthrene, and carbazole. Furthermore, the system can be used to separate durene (l,2,4,5tetra methylbenzene) from C aromatics. In cases where the material to be purified has a relatively high crystallization point, the impure material is raised to a temperature at which only a portion of the constituents are in a crystalline state and the resulting slurry is handled at such a temperature that operation is as described in connection with materials which crystallize at lower temperatures.

The invention is also applicable to the concentration of food products. In the preparation of such concentrated foods, the process consists generally of the removal of water from such products. One special class of foods which can be concentrated in this manner is that of fruit juices, such 4as grape, pineapple, watermelon, apple, orange, lemon, lime, tangerine, grapefruit, and the like. Beverages, such as milk, Wine, beer, coffee, tea, and various liquors can also be concentrated in such a process. The process is also applicable to the concentration of vegetable juices.

Referring now to the drawings, Figure l, a feed stream is passed via a line 1 to a scraped surface chiller 3 by a pump, not shown, which supplies the feed to the process at a constant pressure. The embodiments of this invention which are described in connection with Figure l of the drawings are described with reference to the recovery of para-xylene from a mixed xylene stream. However, it should be understood that the invention is not limited to this system and is equally applicable to the separation of a great number of mixtures as has been described hereinbefore. It is desirable that the crystallizable component in the feed mixture be present in an lamount considerably greater than the eutectic composition of the `feed mixture. Crystals formed from a mixture which is considerably richer in the crystallizable component than the eutectic composition are at la higher temperature than crystals formed from a yleaner mixture and are more easily processed in subsequent purification steps. In the recovery of para-xylene from a feed mixture containing about l5 percent para-xylene, it is necessary to cool the mixture to about -l00 F. in order to crystallize a major portion of the para-xylene present in excess of the eutectic composition. In a feed mixture containing ortho-xylene, meta-xylene and ethylbenzene in addition to para-xylene, the eutectic composition contains about 6 percent paraxylene. However, if the mixture contains about 60 percent para-xylene, it is only necessary to cool it to about F. to crystallize va major portion of the paraxylene present. Although the process of this invention is operable on fairly lean feed mixtures, it is desirable to have a feed which contains the crystallizable component in an amount considerably in excess of the eutectic composition. In the recovery of para-xylene, for example, it is desirable that the feed contain from 30 to 70 percent para-xylene. In order to obtain such a feed for the crystal purification column, it is desirable to employ a preconcentration zone.

The temperature of the feed mixture in chiller 3 is adjusted so as to obtain crystals of at least a portion of the crystallizable component in the mixture. Chiller 3 can be any conventional type chiller which is supplied with refrigeration means that are adequate to adjust the temperature of the mixture as described above. When, for example, a mixture of isomeric lC8 alkyl benzenes is fed through line 1 to chiller 3, that mixture is cooled to a temperature in the neighborhood of 57 to 78 C. A cooling medium is passed via a line 5 through a cooling jacket 7 and out a line 9 to accomplish the aforedescribed adjustment of the temperature of the feed mixture. Chiller 3 also includes a suitable scraper, not shown, which `is rotated by a motor 11 to clean the inner walls of Chiller 3 and to assist in passing the crystal slurry through chiller 3 and out a conduit 13.

The resulting slurry of crystals produced in chiller 3 is passed through the conduit 13 into a Ifilter 15 wherein the non-crystalline material, or mother liquor, is separated from the crystals. Filter 15 is a vacuum filter and, for illustrative purposes, a rotary vacuum filter is shown. Other vacuum filters, such as a belt-type vacuum filter, can also be used. -A constant level 17 of the crystal slurry is maintained in the bottom of filter 15 by a suitable level sensing device, not shown, which controls the rate at which a slurry is introduced to filter 15. The slurry level 17 can, of course, be maintained by hand control or any other suitable arrangement of automatic controls. Filter 15 comprises a rotatable drum 19 which is partially immersed in the slurry of crystals in the bottom of filter 15. Vacuum is applied to drum 19 by conduits 21 and 2.3. As drum 19 rotates through the slurry of crystals in the bottom of -lter 15, a cake of crystals and mother liquor is formed upon drum 19 and mother liquor is removed from this cake through conduits Z1 and 23 by a vacuum pump 25. The liquid mother liquor is passed via a conduit 27 -to a liquid-gas separator 29. The liquid mother liquor is removed from separator 29 via a conduit 31 and entrained gases are removed from separator 29 via a conduit 33 to be returned to filter 15.

Drum 19 includes a filter medium or screen upon its outer surface and upon which the cake of crystals and mother liquor is formed. The screen can be formed from any suitable filtering means, such as cloth, paper, felt, glass fabric, synthetic fabrics or perforated or porous (sintered) metal or any combination of these or other filtering media as is necessary to effectively retain crystalline solids and pass liquid in the filtration of crystal slurries. Drum 19 is totally enclosed in a heavily insulated housing 35. Filter 15 also includes a spring loaded scraper 37 which is provided for removing crystals from rotating drum 19. As drum 19 rotates in a clockwise direct-ion, the crystals are removed by scraper 37 and passed via a conduit 39 to a crystal purification means to be described hereinafter. An electrical vibrator 41 is attached to conduit `39 to aid the passage of the relatively dry crystals through conduit 39.

The crystals are passed through conduit 39 into a chamber 43 wherein the crystals are moved directly into the upstream portion of a crystal purification column 45 by a piston 47. Piston 47 is moved forward by uid flow through a conduit 49 acting against a piston 51 connected to piston 47 by a piston rod 53 to such an extent that the forward face of piston 47 coincides with the side of purification column 45. Piston 47 has a porous face 55 which permits liquid in the crystals in chamber 43 to pass therethrough and out of chamber 43 via a conduit 57.

It is desirable to supply crystals to purification column 45 at a temperature which is suiciently high to prevent freezing of the crystal mass to an impervious plug in column 45 or in chamber 43. To this end, heating Amaterial is passed through a heating jacket 59 which is about chamber 43 by means of an inlet conduit 61 and an outlet conduit 63. During this heating, some of the crystals are melted and, when the crystalline material is compacted in chamber 43 by piston 47, at least a portion of the thus-produced melt is removed via conduit 57 as a result of the compacting.

. Il@ age a Pistcuf?" Pictcfiis'ic Withdrawn by means of fluid tical ilucisli a ccifiduit 67 -ciingraeainst c risica 69 Vwhich Ais connected to piston A6 5 by a piston rod 71.

Y v4121 011.69 is moved in a downstream direction by flow 'rofl-luid througha conduit 7 3'against piston k69. I Piston .5,5 also iilllil 'a -POrQllS facc 75 which permite thc A oughof liquid trom the crystal mass as compat; d and out of column 45 yia a conduit"77.

tliecrystalsVV are .moved downwardly through column'45to a melting zone 4in the lower end of column 45 which includes a heating element79. Heating eleeicel 79`ca bc any type of heating dcvicesucli aslcoilc which a'heat exchange fluidis passed oran eleclieatiug element which may be either within or wilh- Y out chamber'45.v A portionof the melt thus produced is displaced counter-currently to the movement'of crystals and through a portion'of the crystal mass so as to displace impurities` from the crystal mass. VTherimpurities which are displaced yfrom lthe crystal mass are removed through a wall lter 81 and an outlet conduit 83. Y Under some conditions of operation, the concentration of material corresponding tothe puriiied material in the stream removed thrc'nlghY conduit 83,'i.e., the wall filtrate, is higher than its concentration in the feed mixture introducedV to the process via conduit 1. In this event, Y the material re- Vn'roved from column-V45 via conduit 83 is passed via a conduit 85 back to the feed mixture in line 1 wherein it vis utilized to enrich the feed mixture. This results in formation Yof larger'crystals than can be formedV from a feed V mixture 'containing a lower concentration of the desired component. However, if the concentration of the desired component in the stream removed via conduit 83 is low as compared to the freed stream, the Ymaterial removed via conduit l83 may be disposed of as is desired.

' YPuriied product is removed from column 45'througn a conduit 87. The product is passed viarconduit 87 to a surge ltank 89. (A constant level of liquid product is maintained in v89 by a liquid level controlling dc- 491which operates a motorvalve 93 in an outlet line Y 95 vconveys the product from tank. 89- to storage or any other desired use.

In the copending application of S. J. Kolner, Serial 478,658, `December 30, 1954, lnow Patent No. 2,835,598, Vthere is disclosed and claimed an improved. Y system for controlling the purity of the product from the crystalpurifrication column 45V.. In accordance with the invention of application Serial ,No 478,688, the pressure inthecolumn 45 is controlled by controlling the produ-ct withdrawal rate. The eiectiveness of the coun- -tercunrentdisplacement of the impurities Vby reflux in the lower section of column 45 is indicated by the temperature/*ofthe displaced material or its relationship to the temperature ofthe crystals fed .to the purilication column, since any channeling of reflux through the bed causes the temperature of the displaced material to rise. However, inthe normal operation of any given system lthe temperature of the'feed crystals remains substantially con-Y stapt', therefore, a measure of the wall filtrate temperature itself-reflects the temperature differential existing between the Afeed crystals and the displaced material, or wall filtrate. Thus a lte'rnperature sensitive element 97, such as a thermocouple, is positioned in the crystals at the downstream end of chamber 43 which is the inlet to colurnnSf c A similar temperature sensitive element 99 is vdisposed in conduit -83 which carriesthe displaced material-or wallV filtrate from column 45. Temperature sensitive clcmcrits 9.7 and. 9? are ccneccicd by clccirical Y' leads 101,` 103e and 10,5 i0. ,a temPcceicfc-rccpcnsivc back precsurc cciitrcllcr 1107?"- A' mcicr .valve `109 is com acciai im@ ccnfluii 3,7 lcctwccn the. prcduct outlet in .ccluma -45 and surgc tank S2 Ccritroller 1,07 can be @niet the -cQuv-cnticnal tcaipcraiurc ccutrcllcrs and operates motor valve 109 in response .a temperature differential existing bciwccutw@ peints. between elc- .perature of the v crystals-fed ftocolumnAS. When the temperature .ci thc ltraic-itl ccnduit 8 3 ,rises above the preselected limits, controller 10,7 operates -valve 109 to `percui@ greater flow of prducl thrcush'ccaduit 87- iThiS lreduces'the back prcssurc, 01.1; cclurhnft g and I l.c.f cr,cascs the amount of reux'through the crystal bed, thus reducing the filtrate temperature. .When the temperature of clic' ltrclc in conduit. 83 falls bclcw the prcsclcctd limits,

controller 107 operates valve 109 so as to reduce product flow through conduit 87, increasing the back pressure on column 45 thus Itending .to increasethe temperature in ccnduii 83 as channeling conditions are approached. Channeling ic causedbv cXccSSivc rcux- Y In accordance with this invention the rate of flow of non-crystalline material separated in filter `1K5 and passing through conduit 277 is maintained ina preselected ratio to the rate of flow o f either the product in line 9 5 or the feed mixture in -line 1. The preferred embodiment of invention is illustrated in Figure l wherein the rate of ow of non-crystalline material, or mother liquor, ,is maintained in a preselected ratio to Athe rate of flow of product in -line 95. To this end, an orifice plate 111is connected into conduit 27 downstream of the point'at'which conduits 21 and 23 connect into conduit 27. A pair of tubes 1-13 and 115 are connected to plate 111 andV transmit the differential pressure existing across orifice plaie 111 kto a diierential pressure cell 117. Differential pressure cells 'which will perform the f 'unction required are commercially available and one such cell is called a D/P ceILVmanufactured by the Foxboro Company, Foxboro, Massachusetts, and Vis shown on page 23 of vtheir bulletin No. 450.

factured by The Foxboro Company.V An orice plate 123 is connected into line95. A pairof tubes IZS-and Y1 27 convey the differential pressure existing across plate 123 to a differential pressure cell v129. A tube'lvlrconveys a pneumatic signal which is indicative of the differential pressure across orifice plate 123v to controller 121. A

motor valve 133-is connected into conduit ),IfdownstreamV of orifice plate 111. A tube 135.is connected lfrom controller 121y to Vvalve l133. Ratio ow controller V12,1 has an vindex setting which permits controller 121th lbe adjusted so that valver133 is opened and closed by controller 121 sc as tc maintain apresclccted ratio between thenew of material in conduit 27 and line V95 in accordance with the setting of the index. The index vsetting for controller 121 may be set by hand or may beautomatically adjusted .by anotherV instrument which automatically adjusts the Y index of controller 121fas is desired.

Thus, an analyzer 137 is provided to continuously monitor the composition of the feed stream inline 1. A

l sample line 139 c`onnects line 1 to analyzer 137 so that a Vsanipleof the feed is Vpassed through the analyzer and out -aconduit Y1 471. Analyzer V137 Vis connected by apair pneumatic signal in pipe 149 is applied to controller '121 so that the` index setting is continuously adjusted in fcspcticc t9 thc ccm-pcciiicn Qi. the fccd streamen@ vil; irlvcrcc'prcpqrticu with respect tc the proportion, of @esubsequently crystallized component in the feed mixture as reiiected by the output of analyzer 137.

All of the instruments described in this system are commercially available. A suitable analyzer 137 is the Perkin- Elmer Model 93 Bichromator Analyzer manufactured by The Perkin-Elmer Corporation, Norwalk, Connecticut. This instrument is described in vol. 4, No. 2, Winter 1953, of the Perkin-Elmer Instrument News for Science and lndustry and in vol. 5, No. 1, Fall 1953, of the same publication. The latter reference sets forth experimental data resulting from the analysis of para-xylene in a stream of mixed xylenes and ethylbenzene which is the type of feed stream used in my system in connection with the embodiment shown in Figure 1. A suitable controller 147 for converting the electrical output of analyzer 137 to a pneumatic signal is the Brown Electronik Potentiometer Controller, having combined therewith the Air-O-Line pneumatic control mechanism, manufactured by The Brown Instrument Company, Philadelphia 44, Pennsylvania. This instrument is described in their bulletin No. -4, copyrighted 1942. Controller 147 thus produces a pneumatic signal which is indicative of the composition of the feed stream in line 1 and which is supplied to ratio flow controller 121 to continuously adjust its index setting in accordance therewith.

Referring now to Figure 2, modifications of my invention are shown. In the system shown in Figure 2, the crystal slurry produced in chiller 3 is passed through an in-line filter, indicated generally as 151, rather than through a vacuum filter as was shown in Figure l. Thus, the slurry is passed out of chiller 3 and through a conduit 153 into an elongated chamber 155 connected to filter 151. The crystals are moved through in-line filter 151, comprising `a filter medium 157 and an outer jacket 159 by a piston 161. Piston 161 is moved forward by uid flow through a conduit 163 acting against a piston 165 Which is connected to piston 161 by a piston rod 167. Piston 161 does not have a porous face as did piston 47 in the embodiment shown in Figure 1, and therefore, When the crystals are compacted in chamber 155 and filter 151, non- Vcrystalline material in the crystal slurry is expressed from the crystals and removed from filter 151 via a conduit 169. Further movement of piston 161 forward places the forward face of piston 161 so that is coincides with the side of purification column 45 and, thus, crystals in filter 151 are moved into column 45.

The in-line filter shown in Figure 2 is most effective when used to separate crystals from non-crystalline material where the amount of the component of interest in the feed stream is generally over 50 percent of the feed stream. This is because the in-line filter cannot handle as great volumes of non-crystalline material as can the vacuum filter shown in connection with Figure 1. For

. example, a feed stream which is preferred for use in the system of Figure 2 is a commercial cyclohexane stream wherein the cyclohexane generally is about 85 percent by volume of the stream. A commercial cyclohexane stream includes, in addition to cyclohexane, small amounts of such materials as methylcyclopentanc, 2,2-dimethylpentane, 2,4-dimethylpentane, 3,3-dimethylpcntane, 1,1-dimethylcyclopentane, 2,3-dimethylpentane, Z-methylhexane and benzene.

The operation of column 45 to produce a purified product in l-ine 95 and a displaced material, or wall filtrate in line 83 is the same in Figure 2 as that described in connection with Figure 1 yand the same reference numerals are given to identical elements. In accordance with the embodiment shown in Figure 2, however, the rate of flow of non-crystalline material, or mother liquor, removed 'from filter 151 via conduit 169 is maintained in a preselected ratio to the rate of flow of feed entering the system via line 1. To this end, an orifice plate 171 is connected into line 169. A pair of tubes 173 and 175 transmit the differential pressure existing across plate 171 to a differential pressure cell 177. A pneumatic signal which is indicative of the differential pressure across plate 171 is transmitted via a pipe 179 from cell 177 to the ratio flow controller 121. An orifice plate 181 is connected into feed line 1 and the differential pressure existing across plate 181 is transmitted by a pair of pipes 183 and 1:25 to a differential pressure cell 187. A pneumatic signal which is indicative of the differential pressure across plate 181 is transmitted by a pipe 189 from cell 187 to controller 121. A motor valve 191 is connected into line 169 downstream of plate 171. A pipe 193 conveys a pneumatic signal from controller 121 to valve 191 so that the rate of iow of mother liquor in line 169 is maintained in a preselected ratio to the rate of ow of feed to the system via l'me 1. Analyzer 137, acting through controller 147 operates to continuously adjust the index setting of controller 121 in response to changes in composition of the feed stream in the same manner as was described in connection with Figure 1.

To further aid in maintaining a constant solids content in the crystalline material fed to column 45,'the rate of ow of refrigerant to chiller 3 Via conduit 5 is maintained in a preselected ratio to the rate of flow of feed to the system via line 1. To this end, an orifice plate 195 is connected into conduit 5 and is connected by a pair of pipes 197 and 199 to `differential pressure cell 201. A pipe 203 transmits a pneumatic signal from cell 201 to a ratio iiow controller 2135 which is indicative of the differential pressure existing across plate 195, An orifice plate 207 is connected in line 1 and is connected by la pair of pipes 299 and 211 to a differential pressure cell 213. A pipe 215 transmits. la pneumatic signal from cell 213 to controler 265 which is indicative of the differential pressure existing across plate 2117. A motor valve 217 is connected into conduit 5 downstream of plate 195. A pipe 219 conveys a pneumatic signal from controller 205 to valve 217 to open and close valve 217 so that the rate of ow of refrigerant in line 5 to chiller 3 is maintained in `a preselected ratio to the rate of ow of feed in line 1 to the system.

Although the crystal purification column shown in the drawing is operated in a vertical position, it can be operated in any position, including an inverted position. A better understanding of the invention can be obtained by reference to the following example which is intended to exemplify but not unduly restrict my invention.

Example A feed mixture comprising 17 percent by Weight paraxylene, 17.5 percent by weight ortho-xylene, 33.4 percent by weight meta-xylene, 27.5 percent by weight ethylbenzene and 4.6 percent by weight toluene is supplied to the chiller-at the rate of 857 gallons per hour. Liquid filtrate material recovered from column 45 through tilter 81 and conduit 83 contains about 40 percent paraxylene. This stream is returned to chiller 3 via conduit at a rate of 128 gallons per hour so that the total feed to the chiller is 985 gallons per hour. The total feed is cooled to a temperature of M70 C. with the resultant formation of about 15 percent solids. The slurry of mother liquor and crystals is passed to the filter 15 and 769 gallons per hour of mother liquor containing about 6 percent by Weight of para-xylene is removed therefrom via conduit 27. The crystals recovered from filter 15 are introduced into the purification column 45 through chamber 43 in the form of a slurry of crystals and mother liquor. The feed to column 45 is 216 gallons per hour of a slurry containing 40 percent by weight solids.

Compacting piston 65 is operated by a conventional power source such as a hydraulic piston and cylinder in such manner that the compacting stroke requires from 20 to 40 seconds and the retracting stroke requires from 3 to 5 seconds. Heat is added to the end portion of the purification column so as to maintain the heating section at a temperature of approximately 27 C. Displaced material, i.e., Wall filtrate, is withdrawn from the column Temperature vcontroller 107 is adjusted so as to maintain theltrate in conduit 8,3 at a temperature within the .range of 0.5 to 2 C. higher than the temperature of the feed toi thepuriiication column .in chamber43. VWhen the temperature of the 'dltrate in conduit 83 tends to rise above 1 thepredeterminedn limits, controller 1%7 operlates so as to open valve .1709 `and thus lower the pressure on column 4 5. This reduces the flow of reflux, which reduces the tendency to channel, and thereby lowers the Viiltrate, temperature. When the filtrate temperature tends lto fgo -below the minimum limit, the controller 157 operatesrso as to close valve 109 thus increasing the back pres- )to about .40 p.s.i.g. at the end of the compacting stroke,

but is, of course,.substantia`lly reduced during the retracting Vstroke of the piston. Thus, control of the column is vaccomplished during the compacting piston strokes.

VThe product removed through conduit 87 contains about99 percent-by weight para-xylene.

p In accordance with this invention, the solids content in the crystal slurry fed from chamber 43 to column 45 is maintained at a Vconstant level by maintaining a preselected ratio between the rate of llow of mother liquor .in conduit 27 to the rate of flow of either product removed via line 95 or feed to the process via line 1. In the process of -this example, 769 gallons per hour of mother liquor is removed via conduit 27, 88 gallons per hour of product is removed via line 95 and 985 gallons per hour of feed are passed lto the process via line 1. Thus, should either 'the flow of product or Vof feed increase l1i) percent, ratio flow controller 121 opens valve Y133 so 'as to'increase the'llow of mother liquor Vin conduitf27 'by l percent, and should Vthe-flow of product or feed de- 'cre'ase by 10 percent, 'a corresponding decrease in the Vflow of mother liquor is elected. It will be apparent to those skilled in the art that various modications of this invention can be made upon study 'of the accompanying disclosure which will be Vvclearly. within the spirit and scope iof thisl invention.

lclaim: 1.V An apparatus -for' separation Yof'a component of .a

feed mixture stream by fractional crystallization whichY comprises, means for adjusting-.the temperature of said feedmixture stream V'so asto producea slurry of crystals of `said componentfthereof in non-crystalline material I thereof comprising a conduit lmeans (l) for passing aV fluid in heat exchange relationship with 'said -feedmixture', 'conduit means (2) for passing said feed mixtureistream to 'said temperature adjusting means, means for measuring the rate-of flow of Avsaid-ilu'd 'in-'said conduit means Y l), means for measuring therate of flow of said feed mixture V`in 'said conduit means (2'),v means for controlling Said -rate o'f flow of said fluid lin'conduitmeans (l) so'as'to maintain a preselected ratio between the rate'ofllow'of said uid in conduit 'means'V (l) and V the rate of flow-of said'fered mixture in conduit means 2) means for Vseparating 'crystalline and'V non-crystalline/-materiah means Yfor ,passing said slurry to said separating means, an elonmeans through-said vcrystal Yinletto said crystal purificalftionlchambencondiiit means C3) for removingnQn-Qrys- Ytalline material from said separating means, -conduit means (4) for removing purified product `fromrsaid product outlet, means for measuring the temperature differential between said crystal inlet and said wall filtrate outlet,

means for controlling the rate of flow of product in' said conduit means (4) in'respon'se toV said temperature differential, means for measuring the rate (5) of dow of non- `crystalline material in said conduit means (3), means'for measuring the rate (6)V of ow of one of said` product in said conduit means (4) downstream of said means for controlling the rate of flow of product in said conduit means (4) and the rate of flow Vof said feed rmixture in `said conduit means (2), means for controlling said `rate so as to maintain a preselected ratio between said vrate (5) of llow and said rate (6.) of flow, means for analyzing said feed mixture stream in said conduit means (2) compr-is'mg means forgproducing'an output-'signal indica'tive of the amount of said component insaidjfeed stream, and means for applying said output signal to vsaid means for controlling the rate (5) of llow so as to adjust Vsaid preselected ratio between said rate (5)V and said rate (6) in response to the amount of said component in said feed stream.

2.'An Yapparatus according to claim 1 Vwherein said means for measuring said rate (6) of flow is the means for measuring the rate of flow Vof product in said conduit means (4).

3. An apparatus according to claim 1 wherein said means for measuring said rate (6) of llow is the means for measuring the rate of dow of feed mixture in said conduit means4 (2). y

4; In a process for carrying out fractional crystallization so asto supply a crystal slurry having asubstantially 35 Yconstant solids content to a ycrystal puriication zone in 40 y at least a portionof said 'non-crystalline material is removed in said-zone, the remaining crystals yand adhering noncrystalline'material are passed from said zone lto a crystal purification zone, -said Vcrystals are moved as a compactmass throughsaid purification zone V'to a melting zone, apo'rtion of said crystal mass 'is melted, a first portion of resulting-melt isY displaced countercurrently through l cation zone as a product stream, the improvement which comprises controlling theV rate of dow of said product 're- 1 lmoved from the purication zone so as vto maintain a preselected ratio between` the temperature of said displaced VYmaterial removed from said purification zone and the temperature of s'aid'rcrystals and adhering non-crystalline material passed to said crystal purification zone and 'con- 'trolling the rate of dow of said non-crystalline material removed from saidpre-concentration zone so as, to maintain a pre-selected ratio'between the rate ofow of noncrystalline material Aremoved from the pre-.concentration Zone and the rate of dow Aof oneof said streams.

. Y 5. A method according to claim 4 whereinth'e rate of jow of non-crystalline material removed from said pre` concentration zone is controlled so as Ato maintain a preselectedratio between said rate of flow of non-crystalline .flow-of"non-crystalline material removed Ifrom said .pre-

concentration zone is controlled -so as to` maintain apreto said-preconcentration zone.

'n 47. YInV arprocess for-carryingvoutfracfional crystallization so as to supply a crystal slurry having a substantially constant solids content to a crystal purification zone in which process a liquid feed stream mixture is passed to a pre-concentration zone, the temperature of said mixture is adjusted in said zone so as to obtain crystals of a component of said mixture in non-crystalline material thereof, as least a portion of said non-crystalline material is removed in said zone, the remaining crystals and adhering non-crystalline material from said Zone are passed to a crystal purification zone, said crystals are moved as a compact mass through said purification Zone to a melting zone, a portion of said crystal mass is melted, a iirst portion of resulting melt is displaced countercurrently through at least a portion of said crystal mass to thereby displace non-crystalline material from said crystal mass, said displaced material is removed from said purification zone and a second portion of said melt is removed from said purification zone as la product stream, the improvement which comprises controlling the rate of flow of said product removed from the purification zone so as to maintain a preselected ratio (l) between the temperature of said displaced material removed from said purication zone and the temperature of said crystals and adhering non-crystalline material passed to said crystal purication zone, controlling the rate of ow of said noncrystalline material removed from said pre-concentration zone in a ratio dow controlling zone so as to maintain a preselected ratio (2) between the rate of flow of non: crystalline material removed from the pre-concentration zone and the rate of flow of one of said streams, constantly passing a sample of Ksaid feed mixture through an analysis zone, producing a signal in said analysis zone which is indicative of the proportion of said component in said feed mixture and passing said signal to said ratio iiow controlling zone so as to constantly adjust said ratio (2) in inverse proportion With respect to the proportion of said component in said feed mixture.

8. In a process for carrying out fractional crystallization so as to supply a crystal slurry having a substantially constant solids content to a crystal purification zone in which a liquid feed stream mixture is passed to a preconcentration zone comprising a cooling zone and a 'iltering zone, a refrigerant is passed in heat exchange relationship with said feed mixture in said cooling zone so as to cool said feed mixture to obtain a slurry of crystals of a component of said mixture in non-crystalline material thereof, said slurry of crystals and non-crystalline material is passed from said cooling zone to said filtering zone, said slurry is filtered to produce non-crystalline material (A) and crystals (B) of said component and adhering non-crystalline material, said non-crystalline marial (A) is removed from said filtering zone, said crystals (B) and adhering non-crystalline material are passed to a crystal puriiication Zone, said crystals (B) and adhering non-crystalline material are moved as a compact mass through said purification zone to a melting zone, a portion of said crystal mass is melted, a first portion of resulting melt is displaced countercurrently through at least a portion of said crystal mass to thereby displace non-crystalline material from said crystal mass, said displaced material is removed from said purification zone and a second portion of said melt is removed from said purification zone as a product stream, the improvement which comprises controlling the rate of ow of said refrigerant in said cooling zone so as to maintain a predetermined ratio (l) between the rate of dow of refrigerant and the rate of flow of said feed mixture into said preconcentration zone, controlling the rate of fiow of said product removed from the puriiication zone so as to maintain a pre-selected ratio (2) between the temperature of said displaced material removed from said purification zone and the temperature of said crystals (B) and adhering non-crystalline material passed from said filter zone to said purification zone and controlling the rate of flow of said non-crystalline material (A) removed from said filter zone so as to maintain a preselected ratio (3) between the flow rate of non-crystalline material removed from said filter zone and the dow rate of one of said streams.

9. A method according to claim 8 wherein the rate of flow of non-crystalline material removed from said preconcentration zone is controlled so as to maintain a preselected ratio between said rate of iow of non-crystalline material and the rate of flow of product removed from said purification zone.

i0. A method according to claim 8 wherein the rate of flow of non-crystalline material removed from said preconcentration zone is controlled so as to maintain a preselected ratio between said rate of flow of non-crystalline material and the rate of flow of said feed mixture passed to said preconcentrat-ion zone.

1l. In a process for carrying out fractional crystallization so as to supply a crystal slurry having a substantially constant solids content to a crystal puriiication zone in which process a liquid feed stream mixture is passed to a pre-concentration zone, the temperature of said mixture is adjusted in said zone so as to obtain crystals of a component `of said mixture in non-crystaliine material thereof, at least a portion of said non-crystalline material is removed in said zone, the remaining crystals and adhering non-crystalline material from said zone are passed to a crystal purification zone, said crystals are moved as a compact mass through said purification zone to a melting zone, a portion of said crystal mass is melted, a iirst portion of resulting melt is displaced countercurrently through at least a portion of said crystal mass to thereby displace non-crystalline material from said crystal mass, said displaced material is removed yfrom said puriiication zone and a second portion of said melt is removed from said purification Zone as a product stream, the improvement which comprises controlling the rate of flow of said non-crystalline material removed from said pre-concentration zone to maintain a preselected ratio between kthe rate of flow of said non-crystalline material from said pre-concentration zone and the rate of iiow of one of said streams, analyzing said feed and determining therein the concentration of said component, and adjusting said ratio by decreasing the same when said concen- -tration exceeds a predetermined value and increasing the same When said concentration becomes less than a predetermined value.

i2. In a process for carrying out fractional crystallization so as to supply a crystal slurry having a substantially constant solids content to a crystal purification zone in which process a liquid feed stream mixture is passed to a pre-concentration zone comprising a cooling zone and a filtering zone, a refrigerant is passed in heat exchange relationship with said feed mixture in said cooling zone so as to cool said feed mixture to obtain a slurry of crystals of a component of said mixture in non-crystalline materim thereof, said slurry of crystals and non-crystalline material are passed from said cooling zone to said filtering zone, said slurry is filtered to produce noncrystalline material (A) and crystals (B) of said component and adhering non-crystalline material, said noncrystalline material (A) is removed from said iiltering zone, said crystals (B) and adhering non-crystalline material are passed to a crystal purification zone, said crystals (B) and adhering non-crystalline material are moved as a compact mass through said purification zone to a melting zone, a portion of said crystal mass is melted, a first portion of resulting melt is displaced countercurrently through at least a portion of said crystal mass to thereby displace non-crystalline material from said crystal mass, said displaced material is removed from said purification zone and a second portion of said melt is removed from said purification zone as a product stream, the improvement which comprises controlling the rate of iiow of said refrigerant in said cooling zone so as to maintain a predetermined ratio between the rate of flow of refrigerant Y and the rate of ow of said feed mixture into said preconcentration zone, and controlling the'rate of'ow of said non-crystalline material (A) removed from said filter zone Yso as to maintain a preselected ratio Y(3) between the rowrate of non-crystalline material removed Yfrom saidffilter 'zone and the'llow rate of 'oneof 'said streams.

Y `13. An apparatus for separation'of a component of a feed 'mixture stream Vby fractional` crystallization which comprises, means for adjustingthe temperature of said ,Y 'feed mixture stream so as to produce a slurry of crystals *(1), means for measuring the 4rate of flow of said feed mixtureY in said conduit mean (2), means -for controlling t 'said rate of flow of said `fluid in conduit means (l) so as to maintain a preselected ratio vbetweenthe rate Vof ow of said Vfluid `in conduit means (1) and the vrate of liow fof said feed mixtureY in conduit means (2), means for separating crystalline and non-crystalline material, means "for'passing said slurry to said separating means, an elongated crystal purication chamber comprising, a

piston adapted to move crystals therethrough, a crystal inlet, a wall filter, a wall ltrate outlet and a product outlet, means for passing said crystals from said separating means through said crystal inlet to said crystal vpurification chamber, conduit means (3) for removing non-crystalline material from said separating means, con- 16 duit means (4) for removing purified product from said product outlet, means Yfor measuring the frate (5) of iiow of non-crystalline material in V`said "conduit-means (3),

f means for measuring 'the rate (6)-ofj`flow of one of said product in said 'conduit means (4) and saidfeed'mixture in said conduit means (2), means for-controlling said rate (5) so as to maintain a'preselected ratio between said rate (5 of How vand said rate f( 6 of' ow, means `for analyzing said feed mixture stream in said vconduit means (2) comprisingmeans for'produ'cingtan voutput signal indicative of the amount of said component in said feed' stream, and means for applying said output signal t'o`said means for controlling the rater(5')- of ow so as to adjust said preselectedjratio vbetween' said rate (5-)`and said 'rate (6) Yin response to variations in the/:amount of said :component in said feed stream.

14. Apparatus according to claim'3 vand containing the following additional elements inv'combina'tionrzan additional means for measuring the r'ateof ow of said feed mixture in said conduit'means (2),"mea'ns forme'asuring the rate ofow lof uid 'in said conduit 'means (l), and means for maintaining a lpreselectedratio between vsaid last two-mentioned rates Aof flow.-

References Cited the'file of this patent UNITED STATES 'PATENTS 

12. IN A PROCESS FOR CARRYING OUT FRACTIONAL CRYSTALLIZATION SO AS TO SUPPLY A CRYSTAL SLURRY HAVING A SUBSTANTIALLY CONSTANT SOLIDS CONTENT TO A CRYSTAL PURIFICATION ZONE IN WHICH PROCESS A LIQUID FEED STREAM MIXTURE IS PASSED TO A PRE-CONCENTRATION ZONE COMPRISING A COOLING ZONE AND A FILTERING ZONE, A REFRIGERANT IS PASSED IN HEAT EXCHANGE RELATIOHSHIP WITH SAID FEED MIXTURE IN SAID COOLING ZONE SO AS TO COOL SAID FEED MIXTURE TO OBTAIN A SLURRY OF CRYSTALS OF A COMPONENT OF SAID MIXTURE IN NON-CRYSTAL LINE MATERIAL THEREOF, SAID SLURRY OF CRYSTALS AND NON-CRYSTALLINE MATERIAL ARE PASSED FROM SAID COOLING ZONE TO SAID FILTERING ZONE, SAID SLURRY IS FILTERED TO PRODUCE NONCRYSTALLINE MATERIAL (A) AND CRYSTALS (B) OF SAID COMPONENT AND ADHERING NON-CRYSTALLINE MATERIAL, SAID NONCRYSTALLINE MATERIAL (A) IS REMOVED FROM SAID FILTERING ZONE, SAID CRYSTALS (B) AND ADHERING NON-CRYSTALLINE MATERIAL ARE PASSED TO A CRYSTAL PURIFICATION ZONE, SAID CRYSTALS (B) AND ADHERING NON-CRYSTALLINE MATERIAL ARE MOVED AS A COMPACT MASS THROUGH SAID PURIFICATION ZONE TO A MELT- 